The invention relates generally to a flow meter and more specifically concerns accurately monitoring the flow rate of liquids that are released slowly so as to form drops. An example of a use for this invention is the medical use of intravenous liquids usually administered in a gravity feed system with flow control adjusted in a gravity feed system with flow control adjusted by clamping shut a plastic tube and monitoring the resulting drops.
Currently, the monitoring of liquids for intravenous applications is achieved by several means. First, and the most usual method, the drip rate is counted per unit time by watching the drops through a transport tube. The obvious disadvantages of this method are that it depends on the count accuracy of the attendant and is prone to human count error as well as error associated with the drop size variations. Determining the sizes of the drops is essential to accurately monitoring flow rate.
A second and more complex system involves a positive displacement pump which presses the liquid out of a tube using a roller. The flow is controlled by the tube diameter and the roller rotation rate. The positive displacement system is very expensive and must constrict the tube to function correctly. Therefore, the liquid must be able to sustain compressive crushing forces which may impair some liquids such as blood which contains physical cells.
A third group of systems included optical and electrical systems. Some of these systems only count the drops and are therefore subject to error. Some of these systems also measure the drop sizes. However, these systems are complex and subject to error in measuring the drop sizes.
An example of the third group of systems is disclosed in U.S. Pat. No. 4,583,975. In this system, the drops impinge on a surface of accumulated fluid and a piezoelectric element measures the sizes of the drops. Inasmuch as these measurements depend on the height through which the drops fall, it is essential to maintain this height constant at all times. That is the height of the surface of the accumulated fluid must be maintained at the same distance from where the drops are formed. This system does not provide a means for accurately maintaining this height constant and is therefore subject to error.
It is an object of the present invention to accurately monitor the flow rate of liquids that are released slowly so as to form drops.
Another object of this invention is to monitor the flow rate of liquids that are released slowly so as to form drops by accurately measuring the liquid volumes of the drops.
A further object of this invention is to monitor the flow rate of liquids that are released slowly so as to form drops by accurately maintaining a constant height through which the drops fall whereby the liquid volumes of the drops can be accurately measured.
Other objects and advantages of this invention will become apparent hereinafter in the specification and drawing.